102 HISTOLOGY 



premolars at 33 weeks (40 cm.). Each front tooth develops in the alve- 

 olus occupied by the corresponding deciduous tooth, but later a bony 

 septum forms between the two teeth and subdivides the alveolus. When 

 the deciduous teeth are shed, the partitions are resorbed, together with 

 the dentine and cement of the roots of the deciduous teeth. This resorp- 

 tion is accompanied, as in bone, with the production of osteoclasts. 



The portion of the dental lamina which is not utilized in producing 

 enamel organs becomes perforated and forms irregular outgrowths (Fig. 

 91). This disintegration begins in the front of the mouth and spreads 

 laterally. Epithelial remnants from the lamina have been found in the 

 gums at birth and have been mistaken for glands. Like other epithelial 

 remains they occasionally develop abnormally, forming cysts and other 

 tumors. The deepest part of the lamina, below the enamel organs of 

 the permanent teeth, is considered by Rose to be a possible source of a 

 third set, and he states that a case has been reported to him in which 

 such a set, consisting of thirty-two teeth, developed on the lingual side 

 of the permanent teeth. The models which Rose prepared, showing 

 the enamel organs in various stages of development, form the basis of 

 present accounts of tooth development. They are described and well 

 illustrated in the Arch. f. mikr. Anat., 1891, vol. 38, pp. 447-491. 



ENAMEL ORGAN AND ENAMEL. 



The basal cells of the oral epithelium may be followed as a distinct 

 layer over the dental lamina and enamel organ, as shown in Fig. 92. 

 This suggests that the enamel organ should be regarded as an infolding 

 of the oral epithelium, and the occurrence of a transient dental groove 

 immediately above the lamina (Fig. 90, C) favors this interpretation. The 

 basal surface of the epithelium of the enamel organ is therefore directed 

 toward the surrounding mesenchyma, and the superficial cells are found 

 in the interior of the organ. At first these internal cells are in close 

 contact, like those of ordinary epithelium, but later, through an accumula- 

 tion of gelatinous intercellular substance, they constitute a protoplasmic 

 reticulum which resembles mesenchyma, and is known as the enamel 

 pulp (Fig. 93). No vessels or nerves penetrate this pulp. On the side 

 away from the dental papilla the enamel pulp is bounded by the outer 

 enamel cells. At first these are typical cuboidal epithelial cells, but later 

 they become flattened and transformed into a feltwork of pulp fibers. 

 Toward the dental papilla the enamel pulp is bounded by inner enamel 

 cells, which develop differently over the upper and lower parts of the 

 tooth respectively. Over the lower portion of the dental papilla they 

 remain as cuboidal or low columnar cells. Here, through a thinning of 

 the pulp, they are brought into contact with the outer enamel cells, and 



